Antenna system



Aug. 29, 1961 R. A. ORLANDO ANTENNA SYSTEM Filed July 9, 1957 UnitedStates Patent ois Filed July 9, 1957, Ser. No. 670,697 3 Claims. (Cl.343-848) General This invention relates to antenna systems and, thoughnot limited thereto, particularly to omnidirectional antenna systems foruse at UHF and microwave frequencres.

The chief problem in building omnidirectional antennas is one ofobtaining a radiation pattern which is uniform in all directions. Thisarises from the inherent tendency of single radiator elements, even oneso simple as a straight length of wire, to have directional radiationcharacteristics. This limitation has in the past been overcome byutilizing relatively complex arrays of radiator elements, the arraysfrequently being stacked to form several tiers of such arrays. In thismanner the weak points in the radiation pattern of one radiator elementmay be olfset by the strong points in the radiation pattern of one ormore of the other radiator elements.

At higher operating frequencies difficulties are encountered inconstructing complex antenna arrays. For one thing, the more radiatorelements there are, the more diflicult it is to connect them to thetransmission line which feeds the'energy from the transmitter. withoutin troducing impedance mismatches. Such impedance mismatches may causeboth misphasing and unbalance of the magnitudes of the relative currentsdrawn by the different radiation elements which, in turn, causeundesirable alterations in the radiation pattern. It is desirable,therefore, to have an antenna system which requires only a minimum ofactive radiator elements.

Also, strictly as a matter of power efficiency, it is desirable toutilize as few radiator elements as possible because the fewer thenumber of radiator elements, the smaller is the energy loss of theantenna as a whole. For example, resistive losses due to the currentflow within the radiator elements are reduced when the number of suchradiator elements is reduced.

It is an object of the invention, therefore, to provide a new andimproved antenna system of relatively simple construction for obtaininga radiation pattern of increased uniformity.

It is another object of the invention to provide a new and improvedantenna system which has increased power efliciency.

It is a further object of the invention to provide a new and improvedantenna system for obtaining an omnidirectional radiation pattern with aminimum of active radiator elements.

In accordance with the invention, an antenna system comprises an activeradiator element for radiating electromagnetic energy, a surfacepositioned below the radiator element to form a ground plane, and a ringshaped reflector element encircling the radiator element and positionedat a height intermediate the radiator element and the ground plane, theposition being chosen such that energy radiated by the radiating elementtowards the ground plane at a predetermined angle is intercepted andreflected by the reflector element rather than being reflected by theground plane thereby modifying the phase of the energy so that, ratherthan combining in phase opposition with directly radiated energy, andproducing energy null points in space, the reflected energy combines ina phase-aiding manner with directly radiated energy 2,998,605 PatentedAug. 29, 1961 "ice to produce a radiation pattern of increaseduniformity in all directions.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

Referring to the drawing:

FIG. 1 is a plan view of a particular embodiment of an antenna systemconstructed in accordance with the present invention, and 7 FIG. 2 is across-sectional view taken along the section line 22 of FIG. 1.

Description of antenna system Referring to FIG. 1 of the drawing, thereis shown a plan view of an antenna system constructed in accordance withthe teachings of the present invention. Such a system includes an activeradiator element 10 for radiating electromagnetic energy. For the caseof an omnidirectional antenna system, such radiator element 10 may takethe form of a tri-dipole radiator as shown, in which case the radiatorelement includes dipole elements 11, 12 and 13 which are individuallydirectly connected to a center position 14 of the radiator element byway of feed lines 15, 16 and 17, respectively.

The antenna system also includes a nonuniform ground plane having anupper boundary of nonuniform height for modifying the ground planereflections of the radiated energy to obtain a radiation pattern ofincreased uniformity. In the case of the embodiment of the inventionwhich is illustrated in the drawing, such nonuniform ground plane may becomposed of a metallic surface 18 positioned below the radiator element10 to form a conductive ground plane together with a. passive reflectorelement positioned at a height intermediate the radiator element 10 andthe ground plane 18 for effectively altering the height of the groundplane over the region covered by such reflector element. As illustratedin the drawing and best seen in the cross-sectional view of FIG. 2, suchpassive reflector element may take the form of a ringshaped conductiveelement 20, which is centered about a vertical axis 21 corresponding tothe center line of the radiator element 10.

Energy may be supplied to the antenna system, more particularly to theactive radiator element 10, by way of a transmission line conductor 22which may also serve as the vertical support of the radiator element 10.Such support for the radiator element 10 may also include an additionalsupport member 23 made of dielectric material and surrounding thetransmission line conductor 22. The ring-shaped reflector element 20 maybe supported above the ground plane 18 by means of dielectric supportmembers which, for the sake of simplicity, have not been shown. Thelower end of the transmission line conductor 22 which projects throughthe ground plane 18 may be connected to the transmitter by way of asuitable transmission line. Where the antenna is used for receivinginstead of transmitting, such transmission line would instead be coupledto a receiver.

Explanation of operation Considering now the operation of the antennasystem just described, assume the antenna is being used for transmittingpurposes. The radio-frequency energy is supplied from the transmitter byway of the transmission line conductor 22 and the feed lines 15, 16, and17 to k the individual dipole elements 11, 12, and 13. Such di-Considering first the elevational radiation pattern and assuming for themoment that the ring-shaped reflector element 20 of the presentinvention is not present, then the energy radiated to any given point inspace 1 s made up of a direct component indicated by the ray 25 111 FIG.2 and a reflected component indicated by the ray 26 which is reflectedoff the ground plane 18. As is apparent, the distance traveled by thesetwo component rays in reaching a point out in space is different andvaries as a function of the angle 8. Thus at some angles the fieldstrength will be increased due to the in-phase addition of these twocomponents, while at other angles the field strength will be decreasedbecause of the out-ofphase cancellation of these components. Inparticular, at some angles, as seen by an observer in space, these twocomponents will be 180 out-of-phase.

Another useful and perfectly valid 'way of looking at the matter is oneof considering that instead of the ray 26 occurring due to reflectionfrom the ground plane 18, such ray is instead produced by an imaginaryradiator element 10, representing the image of the radiator element 10,which is spaced an equal distance below the ground plane 18. Looking atit in this way and considering the geometry of the matter, it will beapparent that minimums or nulls will occur in the radiation pattern atangles defined by the following relationship:

Sm (i where =the angle of the null 7\=the operating wave length d=theheight of the radiator element 10 above the ground plane 18.

For the case where the radiator element 10 is spaced at a distance ofone wave length above the ground plane 18, the null should appear at anangle of 30. This is the illustrative case which is intended to berepresented in the drawing. As will be seen, the diflerence in pathlength for the rays 25 and 26 is indicated by the dimension A andcorresponds to one wave length. This results in the signal componentsbeing 180 out-of-phase as seen by an observer out in space where it isremembered that the reflected ray 26 experiences an additional 180 phaseshift due to reflection ofl the surface of the ground plane 18.

A feature of the present invention is the recognition of the fact thatcertain regions of the ground plane may be identified with radiation atdifferent elevation angles. As a result, the ground plane regions,corresponding to elevation angles at which nulls or minimums in theradiation pattern occur, may be modified so as to increase the signalstrength at these angles and thereby improve the uniformity of theradiation pattern. For the representative case illustrated in thedrawings, such nonuniformity of the ground plane is introduced byplacing a reflector element, namely the ring-shaped element 20, at aheight intermediate the radiator element 10 and the ground plane 18 andat a position at which it can intercept energy which would otherwise bereflected from the ground plane 18 at the null angle and at a heightabove the ground plane which is sufiicient to modify the phase of suchenergy so that it is no longer in phase opposition with the directradiation indicated by the ray 25.

In other words, for the region of the ground plane 18 covered by thering-shaped reflectorelement 20, that element itself behaves as theground plane and results in a new reflected ray 27 instead of the formerreflected ray 26. The difference in path length for this new reflectedray 27 and the direct ray 25 is indicated by the dimension s. As isapparent from the drawing, this difference in path length s is differentfrom the difference in path length A for the my 26. The effect of theringshaped reflector 20 is to cause the imaginary 01' image 4 radiator10' to appear closer to the ground plane 18 for radiation at the 30elevation angle. As a result, the energy components represented by therays 25 and 27 will no longer be out-of-phase and hence the fieldstrength will have been increased.

Another important feature of the invention is that the uniformity of theradiation pattern is further increased by means of additional energywhich is reflected ofi the inner edge of the back side of thering-shaped element 20. This energy reflected from the inner edge of theback side of the element 20 is indicated by the ray 28 which, asindicated, is thence reflected off the ground plane 18 and radiated outinto space in the same direction as the rays previously considered. Inorder that the energy reflected from the back side of the ring-shapedelement 20 may add to or increase the uniformity of the radiationpattern, it is necessary that the radius from the center axis 21 to theinside edge of the element 20 be proportioned so that the energy pathrepresented by ray 28 will not be 180 out-of-phase with the energyfollowing the direct ray 25.

Considering the resultant energy which will reach the various points inspace, where reflection is occurring off the ring-shaped element 20, theenergy seen by an observer out in space represents the vector sum of thedirect component 25 and of the two reflected components 27 and 28. Forother angles where reflection is occurring primarily off of the groundplane 18 instead of the ringshaped element 20, the energy reaching theobserver will correspond to the vector sum of the direct component, the

rflected component from the back side of the ring, and the reflectedcomponent reflected off of the ground plane 18. Due to the physicalthickness of the different elements, there will be regions intermediatethese two regions where the energy seen by the observer will representthe vector sum of the direct component and portions of all three of thepossible reflected components.

Another feature of the invention is that the energy reflected off theinner edge of the back side or opposite side of the ring-shaped element20, as illustrated by the ray 28, also serves to increase the uniformityof the azimuth radiation pattern. In other words, assuming the observeris at such an azimuth that the field strength would be a minimum due todirect radiation, then the energy reflected from the back side of thering-shaped reflector element 20 may be made to correspond to energywhich would have been directed towards a maximum or peak of the an'muthpatern in the opposite direction.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

1. An omnidirectional antenna system which operates near a ground plane,the system comprising: an active radiator element for radiatingelectromagnetic energy generally in all directions, the energy beingradiated in a given direction on a first side of the radiator elementundesirably being of reduced strength; and a passive reflector elementpositioned at a height intermediate the radiator element and the groundplane but on the opposite side relative to the first mentioned side ofthe radiator element for causing additional phase-aiding energy to bereflected back in said direction on the first side of the radiatorelement to obtain a radiation pattern of increased uniformity.

2. -An omnidirectional antenna system comprising: an active radiatorelement for radiating electromagnetic energy generally in alldirections; a conducting surface positioned below the radiator elementto form a ground plane; some of the energy radiated by the radiatorelement approaching the ground plane at a predetermined angle therewith,which, if reflected by such ground plane, would combine in phaseopposition with energy proceeding directly from the radiator element andthus cause energy null points to be created at certain regions in space;and a ring-shaped conductive element positioned at a height intermediatethe radiator element and the ground plane and centered about a verticalaxis which passes through the center of the radiator element, such thatthe ring-shaped element intercepts energy which would be reflected atsaid predetermined angle and modifies the phase of such energy so thatit is not in phase opposition with energy radiated directly to saidcertain regions in space, thereby to obtain a radiation pattern ofincreased uniformity, the uniformity of the radiation pattern beingadditionally increased in a given direction by the reflection ofadditional phase-aiding energy from the portion of the ring-shapedelement on the opposite side of the radiator element with respect to thegiven direction.

3. An omnidirectional antenna system comprising: an active radiatorelement; a surface positioned below said radiator element to form aground plane; and a ring shaped reflector element encircling saidradiator element and positioned at a height intermediate said radiatorelement and said ground plane, said position being chosen such thatenergy radiated by said radiating element towards said ground plane at apredetermined angle is intercepted and reflected by said reflectorelement rather than being reflected by said ground plane therebymodifying the phase of said energy so that, rather than combining inphase opposition with directly radiated energy and producing energy nullpoints in space, said reflected energy combines in a phase-aiding mannerwith directly radiated energy to produce a radiation pattern ofincreased uniformity in all directions.

References Cited in the file of this patent UNITED STATES PATENTS767,973 Stone Aug. 16, 1904 2,008,931 Schuler July 23, 1935 2,169,553Bruce "a Aug. 15, 1939 2,225,475 Hahnemann Dec. 17, 1940 2,260,273Hahnemann Oct. 21, 1941 2,368,663 Kandoian Feb. 6, 1945 2,440,210 RibletApr. 20, 1948 2,556,046 Simpson June 5, 1951 2,836,820 Pickles et a1 May27, 1958

